Fiberboards, uses and methods of preparation thereof

ABSTRACT

There is provided a fiberboard, such as a MDF fiberboard or a HDF fiberboard, comprising wood fibers, a binder, and a sludge such as a pulp and a paper sludge. The sludge can be, for example, chosen from a primary pulp and paper sludge, a secondary pulp and paper sludge, a de-inking sludge, and mixtures thereof.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority on U.S. provisional applicationNo. 60/822,852 filed on Aug. 18, 2006, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present invention relates to improvements in the field offiberboards or wood panels. In particular, this invention relates tofiberboards that comprise a sludge such as a pulp and paper sludge. Thesludge can be a primary pulp and paper sludge, a secondary pulp andpaper sludge, a de-inking sludge, or mixtures thereof. The presentinvention also relates to processes and compositions for preparing suchfiberboards that comprise one of the previously-mentioned sludges. Forexample, the fiberboards can be medium density fiberboards (MDF) or highdensity fiberboards (HDF).

BACKGROUND OF THE INVENTION

It is nowadays a major concern to considerably reduce the emission ofvarious pollutants in the environment. Moreover, there are presently alot of concerns and social pressure put on waste rejection. The costs ofland burial are high and the cost of fiber is increasing. There is anunbalance regarding supply and demand for woody material.

It would thus be highly desirable to be provided with a solution thatwould permit to obtain fiberboards or wood panels at lower costs andthat would permit to reduce the amount of wood fibers used in thefiberboards or panels production.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a fiberboardcomprising wood fibers; a binder; and a sludge such as a pulp and papersludge. The sludge can be chosen from a primary pulp and paper sludge, asecondary pulp and paper sludge, a de-inking sludge, and mixturesthereof. The fiberboard can be a medium density fiberboard or a highdensity fiberboard.

According to another aspect of the invention, there is provided afiberboard obtained by pressing a mixture comprising wood fibers; abinder; and a sludge such as a pulp and paper sludge. the sludge can bechosen from a primary pulp and paper sludge, a secondary pulp and papersludge, a de-inking sludge, and mixtures thereof. the fiberboard can bea medium density fiberboard or a high density fiberboard.

According to another aspect of the invention, there is provided aprocess for preparing a medium density fiberboard or a high densityfiberboard. The process comprises pressing a mixture comprising woodfibers; a binder; and a sludge such as a pulp and paper sludge, forexample a sludge chosen from a primary pulp and paper sludge, asecondary pulp and paper sludge, a de-inking sludge, and mixturesthereof.

According to another aspect of the invention, there is provided acomposition for use in the preparation of a medium density fiberboard ora high density fiberboard. The composition comprises wood fibers; and asludge such as a pulp and paper sludge, for example a sludge chosen froma primary pulp and paper sludge, a secondary pulp and paper sludge, ade-inking sludge, and mixtures thereof.

According to another aspect of the invention, there is provided acomposition for use in the preparation of a medium density fiberboard ora high density fiberboard. The composition comprises wood fibers; abinder; and a sludge such as a pulp and paper sludge, for example asludge chosen from a primary pulp and paper sludge, a secondary pulp andpaper sludge, a de-inking sludge, and mixtures thereof.

According to another aspect of the invention, there is provided aprocess for treating sludge, the process comprising screening a sludgesuch as a pulp and paper sludge, for example a sludge chosen from aprimary pulp and paper sludge, a secondary pulp and paper sludge, ade-inking sludge, and mixtures thereof, and having a dryness of about0.5% to about 10%, through a screen of 160 μm or coarser in order to atleast partially reduce the amount of ashes contained in the sludge.

According to another aspect of the invention, there is provided aprocess for treating sludge. The process comprises:

-   -   screening a sludge such as a pulp and paper sludge, for example        a sludge chosen from a primary pulp and paper sludge, a        secondary pulp and paper sludge, a de-inking sludge, and        mixtures thereof, and having a dryness of about 0.5% to about        10%, through a screen of 160 μm or coarser; and    -   washing the screened sludge.

According to another aspect of the invention, there is provided aprocess for treating sludge. The process comprises:

-   -   screening a sludge such as a pulp and paper sludge, for example        a sludge chosen from a primary pulp and paper sludge, a        secondary pulp and paper sludge, a de-inking sludge, and        mixtures thereof, and having a dryness of about 0.5% to about        10%, through a screen of 3500 μm or coarser;    -   treating the screened sludge with a desander so as to at least        partially remove the sand contained therein;    -   screening the desanded sludge through a screen of 160 μm or        coarser; and    -   washing the screened sludge.

It was found that by incorporating such a sludge in fiberboards (forexample MDF and/or HDF fiberboards) by partially replacing the woodfibers by sludge, it was possible to produce a fiberboard, at lowercosts, which demonstrated characteristics and properties similar to astandard fiberboard. These types of sludge can thus decrease the rawmaterial costs (wood fibers) and generate a gain on the specific energyapplied since they are already refined. For the paper mills, such atechnology is very interesting since it can considerably reduce oreliminate the costs associated with management of such sludge disposal,for example land burial.

The wood fibers can be virgin wood fibers, post-consumption wood fibers,or a mixture thereof.

The sludge can be present in the fiberboard in an amount of at least 1%,at least 2%, at least 5%, at least 10%, at least 20%, or at least 30%,by weight, based on the total dry weight of the fiberboard.Alternatively, the sludge can be present in the fiberboard in an amountof about 1% to about 40%, about 2% to about 30%, about 4% to about 15%,or about 5% to about 10%, based on the total dry weight of thefiberboard.

The sludge can be obtained from kraft pulping, mechanical pulping orthermomechanical pulping. The sludge can be a primary pulp and papersludge. It can also be a mixture of primary pulp and paper sludge and asecondary pulp and paper sludge. The sludge can comprise about 50 toabout 80% of a primary pulp and paper sludge and about 50 to about 20%of a secondary pulp and paper sludge, based on the total dry weight ofthe fiberboard. The sludge can be a substantially untreated primarysludge taken from a paper mill.

The sludge can have a silica content, which is less than 30%, less than20%, less than 10%, or less than 5%, by weight, based on the total dryweight of the sludge.

Alternatively, the sludge can have a silica content which is about 1% toabout 30%, or about 1.5% to about 25%, by weight, based on the total dryweight of the sludge. The sludge can have an ashes content, which isless than 30%, less than 20%, less than 10%, less than 5%, less than 2%,less than 1%, by weight, based on the total dry weight of the sludge.Alternatively, the ashes content can be about 0.25% to about 30%, about0.5% to about 25%, or about 1% to about 15%, by weight, based on thetotal dry weight of the sludge.

The mixture or composition used for preparing the fiberboards can bedried before being pressed. Before being pressed, the mixture can bedried so as to have a moisture content of less than 15%, less than 12%,or less than 10%. Alternatively, the moisture content can be of about 7%to about 15%, or about 8% to about 12%. The wood fibers, the binder andthe sludge can be mixed together in order to obtain the mixture, beforedrying the mixture. The binder can be mixed with a compositioncomprising the wood fibers and the sludge in order to obtain themixture, before drying the mixture. The wood fibers can be refinedbefore being mixed with the binder and/or sludge. The wood fibers andthe sludge can be mixed together and then refined before being mixedwith the binder.

In the compositions of the present invention, the sludge can be presentin an amount of at least 0.3%, at least 0.5%, at least 0.7%, at least1%, at least 2%, at least 5%, at least 10%, at least 20%, or at least30% by weight, based on the total dry weight of the composition.Alternatively, the sludge can be present in the composition in an amountof about 1% to about 40%, about 2% to about 30%, about 3% to about 20%,about 4% to about 15%, or about 5% to about 10%, based on the total dryweight of the composition. The composition can comprise an antibacterialagent. The antibacterial agent can be present in the composition in anamount of at least 0.1%, or at least 0.5% based on the total dry weightof the composition. The agent can also be present in an amount of about0.1% to about 1.0%.

The fiberboards can have an internal bond strength of at least 0.35kN/mm², at least 0.4 kN/mm², at least 0.65 kN/mm², at least 0.80 kN/mm²,at least 1.30 kN/mm², at least 1.40 kN/mm², at least 1.50 kN/mm², atleast 1.70 kN/mm², at least 1.80 kN/mm², at least 1.85 kN/mm², at least1.90 kN/mm², or at least 1.95 kN/mm². Alternatively, the fiberboards canhave an internal bond strength of about 0.35 kN/mm² to about 1.95kN/mm², about 1.3 kN/mm² to about 1.98 kN/mm² about 1.4 kN/mm² to about1.95 kN/mm², about 1.45 kN/mm² to about 1.85 kN/mm² about, or about 0.40kN/mm² to about 1.1 kN/mm².

The fiberboards can be high density fiberboards having a density of atleast 800 kg/m³, at least 825 kg/m³, at least 850 kg/m³, at least 875kg/m³, or at least 900 kg/m³. Alternatively, the fiberboards can have adensity of about 800 kg/m³ to about 1450 kg/m³, about 800 kg/m³ to about950 kg/m³, or about 850 kg/m³ to about 950 kg/m³.

The fiberboards can be medium density fiberboards having a density of atleast 500 kg/m³, at least 600 kg/m³ or at least 735 kg/m³.Alternatively, the fiberboards can have a density of about 500 kg/m³ toabout 800 kg/m³ or of about 600 kg/m³ to about 800 kg/m³

The binder can be, for example, chosen from formaldehyde-based resins,isocyanate-based resins, and mixtures thereof. The binder can also be aformaldehyde-based resin chosen from urea-formaldehyde resins,phenol-formaldehyde resins, melamine-urea-formaldehyde resins, andmixtures thereof. Alternatively, the binder can bepolymethyldiisocyanate, a phenol-formaldehyde resin or anurea-formaldehyde resin.

The fiberboards can further comprise an antibacterial agent. Theantibacterial agent can be present in the fiberboard in an amount of atleast 0.1%, or at least 0.5% based on the total dry weight of thefiberboard. Alternatively, the antibacterial agent can be present in theboard in an amount of about 0.1% to about 1.0%. The antibacterial agentcan be, for example, chosen from sodium hypochlorite, hydrogen peroxide,sodium benzoate, sodium azide, and mixtures thereof.

As previously indicated, the sludge can be treated before being used forpreparing fiberboards. In fact, the sludge can be, before being passedthrough a screen of 160 μm or coarser, being screened through a screenof 3500 μm or coarser. The process can further comprises passing a waterstream in a counter-current manner through the sludge so as tofacilitate desanding the sludge. The sludge can be washed by maintainingits dryness at about 0.5% to about 3.5%. The sludge can be washed atleast once with water having a temperature of about 40° C. to about 80°C. The sludge can be agitated while being washed. The sludge beforebeing screened, can be diluted with water having a temperature of atleast 50° C. and it can then be agitated, thereby obtaining the solidcontent in solution of about 0.5% to about 10%.

The sludge can also be treated so as to selectively remove clay and/orfines of the type P200 from the sludge. The clay and/or fines, forexample of the type P200, can be removed from the sludge by treating thesludge with a centrifugal strainer, a centrifugal screen, a pressurescreen, or a pressure inclined-screen.

The sludge (treated or not) can be mixed with wood fibers so as toobtain a mixture and then, refining the mixture. The refined mixture canbe mixed with a binder so as to obtain a composition. Such a compositioncan also be dried and then it can be compressed so as to obtain thedesired fiberboards.

According to another embodiment of the present invention, there isprovided in a medium density fiberboard or a high density fiberboardmade by pressing a mixture comprising wood fibers and a resin, theimprovement wherein at least a portion of the wood fibers are replacedwith a sludge chosen from a primary pulp and paper sludge, a secondarypulp and paper sludge, a de-inking sludge, and mixtures thereof.

According to another embodiment of the present invention, there isprovided in a medium density fiberboard or a high density fiberboardcomprising wood fibers and a resin, the improvement wherein at least aportion of the wood fibers are replaced with recycled fibers obtainedfrom a sludge chosen from a primary pulp and paper sludge, a secondarypulp and paper sludge, a de-inking sludge, and mixtures thereof.

BRIEF DESCRIPTION OF DRAWINGS

In the following drawings, which represent by way of example only, someembodiments of the invention:

FIG. 1 is a schematic representation illustrating how are preparedfiberboards according to one aspect of the present invention;

FIG. 2 is schematic representation illustrating a process, according toanother aspect of the present invention, for treating a sludge thatrequires some treatment before using it in the manufacture offiberboards; and

FIG. 3 is schematic representation illustrating a process, according toanother aspect of the present invention, for treating a sludge thatrequires some treatment before using it in the manufacture offiberboards.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Further features and advantages of the invention will become morereadily apparent from the following description of some embodiments asillustrated by way of examples only in the appended drawings wherein:

The following non-limiting examples further illustrate the invention.

EXAMPLES

In the present invention, the sludge used can be used as is, i.e.without requiring any treatment or purification. Alternatively, in somecases the sludge can be optionally treated before being used for theproduction of fiberboards. For example, the sludge can be treated inorder to reduce and/or eliminate undesired odours, reduce and/oreliminate microorganisms, reduce and/or eliminate silica, reduce and/oreliminate fines.

When preparing fiberboards, the gluing step can be carried out at thesame time or just after the refining step. In this case, sludge can beadded before refiner(s). But if it is not the case, sludge can be addedafter refiner(s) if the sludge's parameters allow it. In accordance withthe sludge source, they may contain coarse fibrous particulars, calledshavings, which can be refined. Use of coarse sieves allow to withdrawthese sludge particles to refine them with fibers. This has been testedwith a sieves of 3540 μm opening and then washed two times. Shavings canbe withdrawn at first if grit removal is necessary.

As previously indicated, the sludge can be optionally treated beforeusing it in the production of fiberboards. The following possibilitiescan thus be applied. If a treatment is necessary to decrease the ashcontent (which comprises silica) due to clay and/or sand, one of thethree following methods can be used.

A. Screening of the Sludge:

This method comprises screening the sludge at a consistency of about 1%to about 5%. For example, a screen opening of 160 μm can be used. Thistreatment allowed to decrease ash content from 27% to 8.7% on sludge.The filtrate was disposed.

B. Screening and Rinsing the Sludge

Such a method is similar to the method schematically represented in FIG.2. This method comprises screening the sludge at a consistency of about1 to about 5%. The sludge was rinsed with tap water few times. Betterresults were obtained with 2 rinses. For example, the screen openingused was 160 μm (sieve). Efficiency was increased when sludge wasdiluted to consistency of about 1% with hot water (60° C.) and agitatedduring 5 minutes. This treatment allowed to decrease ash content from27% to 0.7% on sludge. The filtrate was disposed (reject). Fiberscontaining shavings were recovered.

C. Removing Sand, Rinsing and Screening the Sludge

This method, which is similar to the method schematically represented inFIG. 3, comprises removing sand and bark from sludge using a desander.After sand removal, sludge is screened at an opening of 160 μm (sieve)and then, rinsed with tap water few times. Very interesting results wereobtained when rinsing 2 times. Such a step permits to efficiently removeinteresting quantities of fines and clay. Optionally, before passing thesludge in the desander, it is possible to pass it into a coarse sieve(3500 μm or coarser) in order to substantially remove shavings.

Alternatively, clay and fines could also be removed with a centrifugal,a pressurized screen, a pressurized inclined screen or bycentrifugation.

Addition of an oxidative agent such as sodium hypochlorite at 0.5% canbe made so as to stabilize the sludge during at least 96 hours. Combinedto a bacteriostat agent, the period can be extended to 7 days. Afterhaving simulated thermal treatments, fresh sludge had a bacterialcounting similar to standard MDF panel counting. It was shown that theaddition of an oxidative agent was not necessary depending upon storageconditions.

Some results demonstrated that sodium hypochlorite at 0.1% v/v ofconcentration has a bactericide effect on short term (48 hours). Whileat concentration at 0.2% v/v, sodium hypochlorite allows sludge to keepfor 7 days a bacterial counting lower than the factory raw material.

The results shown in Table 1 demonstrated that the amount ofmicroorganisms is not higher in a sludge-containing panel as opposed toa standard panel. The Table 1 shows microbiological results fordifferent steps in the panel production.

TABLE 1 Yeast and Totals molds coliforms E. coli Entérococcus HBAA CFU/gCFU/g CFU/g CFU/g Counting Contaminants Primary sludge 100%  #1 865 405<3 81 11 300   Bacillus positive GRAM, negative #2 250 280 <3 30 14 150  oxidase, positive catalase. Filamentous negative GRAM, negativeoxidase, positive catalase. After refiner 0% #1 <10 <10 <10 <10 39 800  Bacillus positive GRAM, negative #2 <10 <10 <10 <10 39 200   oxidase,positive catalase. 5% #1 <10 <10 <10 <10 25 900   Bacillus positiveGRAM, negative #2 10 <10 <10 <10 23 900   oxidase, positive catalase.10%  #1 <10 <10 <10 <10 7000  Bacillus positive GRAM, negative #2 10 <10<10 <10 9300  oxidase, positive catalase. At former 0% #1 <10 <10 <10<10 <10 None #2 <10 <10 <10 <10 <10 5% #1 <10 <10 <10 <10 <10 None #2<10 <10 <10 <10 <10 10%  #1 30 <10 <10 <10 <10 None #2 <10 <10 <10 <10<10 Rough panel 0% #1 10 <10 <10 <10 <10 None #2 10 <10 <10 <10 <10 5%#1 <10 <10 <10 <10 <10 None #2 <10 <10 <10 <10 <10 10%  #1 10 <10 <10<10 <10 None #2 10 <10 <10 <10 <10 HBAA = Heterotrophic bacteria aerobicand anaerobic facultative

It should be noted that in Table 1, primary sludge results correspond toCFU/g anhydrous on sludge. In all other cases, units are CFU/g humid.This explains detection threshold of 3 CFU/g for primary sludge and 10CFU/g for all others.

In the following examples, which refer to the preparation offiberboards, the latter have been characterized using standard methodssuch as:

Internal Bonds: ASTM D1037-99 Suface bonds: ASTM D1037-99 Modulus ofRupture (MOR): ASTM D1037-99 Modulus of Elasticity (MOE): ASTM D1037-99Stiffness: ASTM D1037-99 Thickness Swelling: ASTM D1037-99 ThicknessEdge Swelling: EN 13329-2000

Example 1

A process as shown in FIG. 1 was carried out in order to produce panelsor fiberboards. FIG. 1 thus schematically represents the process thatwas carried out. The sludge used came from a pulp and paper factory,which produces newspaper with thermomechanic pulp without addition ofclay. Primary and secondary sludges were mixed before being pressed.Primary sludge represents 63% and secondary sludge 37%. Sludgescontained 74% of water. No treatment and no biocide have been done onsludge.

The sludge was mixed with wood fibers in a half and half proportion(anhydrous weight) before being stored in a silo. The wood fiberscontent was about 50% hardwood and 50% softwood. Sludge proportioninjected in the process was dosed with metering screws. Sludgeincorporation tests were at 5% and 10% by weight, based on the total dryweight of the fiberboard.

Sludge and fibers were stepped forward to the predigester and thedigester. Then, they were refined. During the test, specific energy ofthe refiner drop-off from 348 kW/T to 276 kW/T.

A mixture comprising the fibers, the sludge, an urea-formaldehyde resin(16% by weight based on the dry weight of the wood fibers), asteochiometric excess of urea (0.9% by weight based on the dry weight ofthe wood fibers), and wax/paraffin (1% by weight based on the dry weightof the wood fibers) was stepped forward to the former (inlet of thepress) and finally, through the continuous press (Siempelkamp™). Theso-formed products were HDF panels having a 6.6 mm thickness and adensity of 900 kg/m³. Fiber pH increased from 5.23 to 5.34 during sludgeinsertion of 5%.

Internal bonds strength was 1.91 kN/mm² for the control panel, 1.81kN/m² for the panels with a sludge content of 5% and 1.64 kN/mm² for thepanels having a sludge content of 10%.

Example 2

This example was also carried out as shown in FIG. 1. The sludge usedcame from a pulp and paper factory which produces newspaper withthermomechanic pulp without addition of clay. Only primary sludge wasused. The sludge contained 73% of water. No treatment and no biocidehave been done on sludge.

The sludge was mixed with wood fibers in a half and half proportion(anhydrous weight) before being stored in a silo. The wood fiberscontent was about 40% hardwood and 60% softwood. The sludge proportioninjected in the process was dosed with metering screws. Sludgeincorporation tests were at 5.7% and 10% by weight, based on the totaldry weight of the fiberboard of the final mixture. Sludge and fibers arestepped forward to the predigester and the digester. After, they wererefined. During the test, specific energy of the refiner was constant.

Fiber and sludge were stepped forward to the former ((inlet of thepress) and into the continuous press. The product was HDF panels of 6.6mm thickness and a density of 900 kg/m³. Average length of fiber onlywas 0.726 mm and sludge only was 0.583 mm. During the test, the averagelength of mixture with sludge content of 5.7% was 0.686 mm and was 0.688mm for mixture with sludge content of 10%. Table 2 represents resultsobtained for rough panel and Table 3 represents results obtained forsanded panels. The test duration was 3 hours.

TABLE 2 Average internal Average  Minimal bonds Average  surface corePress Mat strength density density density speed moisture Panel (N/mm²)(kg/m³) (kg/m³) (kg/m³) (mm/sec) (%) Control 1 968 930 1 118 870 285 9.05.7% of sludge 1 972 911 1 119 824 285 9.0  10% of sludge 1 903 935 1153 865 285 9.2

TABLE 3 Average internal Minimal bonds Average Average core AverageAverage Edge Heaving Water strength density surface density MOR MOEswelling ASTM absorption Panel (N/mm²) (kg/m³) density(kg/m³) (kg/m³)(N/mm²) (N/mm²) (%) (%) (%) Hardness Control 2 063 882 1 088 818 48 5133.0 16.46 9.58 12.01 9 757 5.7% of sludge 1 849 879 1 090 820 38  4291.0 17.12 9.17 11.85 9 814  10% of sludge 1 503 882 1 117 837 43 4885.0 16.67 9.49 12.74 9 320

Example 3

Example 3, was carried out in a similar manner as shown in FIG. 1. Thesludge used came from a pulp and paper factory which produces newspaperwith thermomechanic pulp without addition of clay. Primary sludge onlywas used. Sludges contained 70% of water. No treatment and no biocidehave been done on sludge.

In the panel factory, the sludge was mixed with wood fibers inproportion of half and half (anhydrous weight) before being stored in asilo. The wood fibers content was about 20% hardwood and 80% softwood.Sludge proportion injected in the process was dosed with meteringscrews. Sludge incorporation tests was 6% by weight, based on the totaldry weight of the fiberboard. Sludge and fibers were stepped forward tothe predigester and the digester. After, they were refined.

Fiber and sludge were stepped forward to the former (inlet of the press)and finally, into the continuous press. The product was HDF panels of7.6 mm thickness and a density of 850 kg/m³. Large amount of watercontained in sludge forced to slow down press speed from 310 mm/s to 265mm/s during the test. The temperature of air dryer was increased of 10°C. Average length of sludge fiber was 0.578 mm and 0.616 mm for fiberonly. Table 4 represents results obtained for rough panels and Table 5represents results obtained for sanded panel.

TABLE 4 Average internal Average Minimal bonds Average surface core Matstrength density density density Press speed moisture Panel (N/mm²)(kg/m³) (kg/m³) (kg/m³) (mm/sec) (%) Control 1 450 849 1 067 753 303 9.46.0% of 1 582 851 1 133 734 298 9.4 sludge Specification min 1 400 850 ±5% min 1 075 min 730 280 8.5-10.5

TABLE 5 Average internal Average Minimal bonds Average surface coreAverage Average Edge Heaving Water Silicate strength density densitydensity MOR MOE swelling ASTM absorption content Panel (N/mm²) (kg/m³)(kg/m³) (kg/m³) (N/mm²) (N/mm²) (%) (%) (%) Hardness (%) Control 1 716856 1 125 737 44.4 5255.6 15.55 7.66 11.85  — 0.041  6.0% of 1 597 851 1125 742 49.5 5733.3 14.56 7.29 12.102 — 0.0586 sludge Specification min1 400 850 ± 5% min 1 075 min 730 min 38 min 4 500 max 16 max 8 — — —

As it can be seen from Tables 2 to 5, it was clearly demonstrated thatthe obtained fiberboards have properties which are substantially thesame than conventional fiberboards (control). In fact, the valuesobtained for the various parameters tested in Tables 2 to 5 aresubstantially the same for fiberboards which include the sludge and forthe conventional fiberboards.

In view of Examples 1 to 3 related to HDF fiberboards, the personskilled in the art would clearly understand how to prepare MDFfiberboards. In fact, it is well known in the present art that one themain differences between preparation of HDF and MDF reside in thepressure applied to the fiberboards i.e. HDF fiberboards require morepressure since they have a higher density. The person skilled in the artwould also understand that various parameters will be modified dependingon the final desired characteristics of the produced fiberboards.

Example 4

In Example 4, a sludge having a water content of 95.17% and ash contentof 25.6% (combustion ash at 525° C.) was used. It was a primary sludgeonly and this sludge came from a thermomechanical process.

The process carried out in Example 4, was similar to the processschematically represented in FIG. 2. The sludge was diluted to 1% solidweight, vigorously stirred during 5 minutes, screened with an opening of3.6 mm and rinsed twice. After, having vigorously stirred the filtrateduring 5 minutes, it was screened through openings of 160 μm and rinsetwice. The screen with openings of 3.6 mm holded 9.8%. Of the solute.These shavings had an ash content of 1%. The screen with an opening of160 μm holded 53% of solid. These fibers had an ash content of 13.5%.This high value was partially caused by sand. The treated sludge wasthen ready to be used in the preparation of fiberboards.

Example 5

In Example 5, the sludge used was the same than in Example 4. Theprocess carried out in Example 5, was similar to the one schematicallyrepresented in FIG. 3. The sludge was diluted to 1% solid weight,vigorously stirred during 5 minutes, screened through openings of 3.6 mm(coarse sieve) and rinsed twice. The shavings (removed) represented 7.6%of solid. They were characterized by 1.21% of ash content. After thefiltrate was vigorously stirred during 5 minutes and the sand wasremoved with a desander. The reject was sand and small bark particles.They represent 7.19% of solid weight with an ash content of 1.1%. Thesupernatant was screened with openings of 160 μm (sieve) and then,rinsed twice. The retentate was 37.4 g of solid with a ash content of11.5%. The treated sludge was then ready to be used in the preparationof fiberboards.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodifications and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice within theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth, and as follows in the scopeof the appended claims.

1. A fiberboard comprising: wood fibers; a binder; and a pulp and papersludge, wherein said fiberboard is a medium density fiberboard or a highdensity fiberboard.
 2. The fiberboard of claim 1, wherein said woodfibers are virgin wood fibers, post-consumption wood fibers, or amixture thereof.
 3. The fiberboard of claim 1, wherein said sludge ispresent in said fiberboard in an amount of at least 1% by weight, basedon the total dry weight of the fiberboard.
 4. The fiberboard of claim 1,wherein said sludge is present in said fiberboard in an amount of atleast 5% by weight, based on the total dry weight of the fiberboard. 5.The fiberboard of claim 1, wherein said sludge is present in saidfiberboard in an amount of at least 10% by weight, based on the totaldry weight of the fiberboard.
 6. The fiberboard of claim 1, wherein saidsludge is present in said fiberboard in an amount of about 1% to about40%, based on the total dry weight of the fiberboard.
 7. The fiberboardof claim 1, wherein said sludge is present in said fiberboard in anamount of about 4% to about 15%, based on the total dry weight of thefiberboard.
 8. The fiberboard of claim 1, wherein said fiberboard hassubstantially the same properties than a conventional fiberboard.
 9. Thefiberboard of claim 1, wherein said sludge is chosen from a primary pulpand paper sludge, a secondary pulp and paper sludge, a de-inking sludge,and mixtures thereof,
 10. The fiberboard of claim 3, wherein said sludgeis a primary pulp and paper sludge.
 11. The fiberboard of claim 3,wherein said sludge is a mixture of primary pulp and paper sludge and asecondary pulp and paper sludge.
 12. The fiberboard of claim 1, whereinsaid sludge is a substantially untreated primary sludge taken from apaper mill.
 13. The fiberboard of claim 1, wherein said fiberboard hasan internal bond strength of at least 1.30 kN/mm².
 14. The fiberboard ofclaim 9, wherein said fiberboard has an internal bond strength of atleast 1.40 kN/mm².
 15. The fiberboard of claim 1, wherein saidfiberboard has an internal bond strength of at least 1.70 kN/mm². 16.The fiberboard of claim 1, wherein said fiberboard has an internal bondstrength of at least 1.90 kN/mm².
 17. The fiberboard of claim 7, whereinsaid fiberboard has an internal bond strength of about 1.3 kN/mm² toabout 1.98 kN/mm².
 18. (canceled)
 19. The fiberboard of claim 17,wherein said fiberboard is a high density fiberboard having a density ofat least 800 kg/m³.
 20. The fiberboard of claim 1, wherein saidfiberboard is a high density fiberboard having a density of at least 900kg/m³.
 21. The fiberboard of claim 3, wherein said fiberboard is a highdensity fiberboard having a density of about 800 kg/m³ to about 950kg/m³.
 22. The fiberboard of claim 1, wherein said binder is chosen fromformaldehyde-based resins, isocyanate-based resins, and mixturesthereof.
 23. In a medium density fiberboard or a high density fiberboardcomprising wood fibers and a resin, the improvement wherein at least aportion of the wood fibers are replaced with recycled fibers obtainedfrom a sludge chosen from a primary pulp and paper sludge, a secondarypulp and paper sludge, a de-inking sludge, and mixtures thereof.
 24. Afiberboard obtained by pressing a mixture comprising wood fibers; abinder; and as a pulp and paper sludge.
 25. A process for treatingsludge, said process comprising screening a pulp and paper sludge havinga dryness of about 0.5% to about 10%, through a screen of 160 μm orcoarser, in order to at least partially reduce the amount of ashescontained in said sludge.
 26. The process of claim 25, furthercomprising mixing the treated sludge with wood fibers and a binder so asto obtain a composition, drying said composition, and pressing saiddried composition so as to obtain a medium density fiber board or a highdensity fiberboard.